GLSL spec 4.1.7 says "Samplers are not allowed to be operands in expressions".
But piglit/tests/glslparsertest/shaders/dataType19.frag uses sampler in constructor expression, and it gets passed in current mesa:
uniform sampler1D s;
int i = int(s); // conversion not allowed
Fixed with glsl2 merge.
verified with mesa master d442a01ac14382d83cdaac87d2832315ceb3e963.